Study Of Influencing Factors On The Properties Of Soy Protein Isolate Emulsion Gels Induced By CaSO₄

Tofu,as a Chinese traditional food,is very important in the food culture of our country.And in some East Asian countries,such as Japan and Korea,tofu is also an essential food for people's diets.Traditional tofu-making is a complex process and usually involves many procedures,for example,soaking,grinding beans in water,filtering,heating,coagulation,breaking the curd,finally pressing and reforming the gel.It is time consuming and moreover,bean dregs and tofu whey emissions during the process will cause environmental pollution and waste of resources.In addition,traditional tofu made from soymilk contains a certain amount of soybean isoflavones and oligosaccharides,which have been considered as source of allergens and flatulence factors to some people.Therefore,using soy protein isolate?SPI?as the raw materials of filled?packed?tofu has gained considerable attentions and compared with traditional process,making tofu with SPI is much simpler,cleaner,and more controllable.Study on the properties of SPI emulsion-based tofu-type gels is of much value and can provide new ideas and important information for tofu-making industry.So far,many studies are available for SPI emulsion gel,but most researchers used Glucono-?-lactone?GDL?and Transglutaminase?TGase?as coagulants,and rare investigations focused on the CaSO₄-induced gel.When compared with traditional tofu,CaSO₄-induced SPI emulsion gel exhibit weak texture and bad water-holding capacity.And the research base of existing researches and SPI emulsion gelation theory is difficult to solve the problems.Therefore,this paper focuses on investigating the various factors influencing SPI emulsion gel properties and providing valuable imformation for the green manufacturing of soy products.Firstly,the influences of soybean variety and coagulant type on the gel properties of SPI emulsions were investigated.The textural,rheological and microstructural properties of eight SPI?extracted form 8 soybean varieties harvested in different regions?emulsion gels induced by CaSO₄,GDL and TGase were determined,and results showed that the textural properties of SPI emulsion gels were significantly related to the amino acid and protein subunits.For CaSO₄-and GDL-induced SPI emulsion gels,the gel strength was positively correlated with the content of 11S fractions and the ratio of 11S to 7S?11S/7S?.And however,the strength of SPI emulsion gels induced by TGase mainly depended on the content of glutamic acid and lysine in SPI,more glutamic acid and lysine resulted in stronger gels.In all samples,CaSO₄-induced SPI?Taiwan292?emulsion gel had the highest strength.The rhetorical data showed that SPI emulsion gel induced by TGase exhibited better elasticity and resilience when compared with CaSO₄-and GDL-induced gels.The microstructure of SPI emulsion gel induced by CaSO₄ appeared coarser than the others,however,most oil droplets were sufficiently entrapped in the gel matrix.In the gelation process of SPI emulsion induced by GDL,large fluctuation in pH led to the destabilization of the SPI emulsion and oil droplets coalescence,as a consequence,the“filling effect”of oil droplets was weakened as“active fillers”,thus decreasing the final gel strength.In the TGase case,the gel appeared as an intermediate between that of CaSO4 and GDL,which had the most uniform structure.Taking all the aspects into consideration,including texture and taste,we chose Taiwan 292 and CaSO₄ as the source of SPI and coagulant,respectively,in the follow-up studies.Protein aggregation is an important factor influencing gel properties,we studied the effects of protein aggregate properties?such as size and content?on the gelation of SPI emulsion.The protein aggregates with different sizes were prepared by heating different concentrations of SPI dispersions.When compared with native SPI,heat treatment at 95°C for 15 min markedly increased the volume-mean diameter of SPI?p<0.05?,and with the increase in the heating concentration of SPI dispersions,the protein aggregate size increased.The hydrophobicity of SPI first increased but then decreased as the SPI concentration at heating increased.Heating a SPI dispersion could unfold the structure of protein molecular and expose hydrophobic groups,therefore hydrophobicity of SPI increase.However,when the SPI concentration at heating was too high,high degree of protein aggregation reached,most hydrophobic groups were involved in the formation of aggregates,leading to the decrease in the SPI hydrophobicity.In addition,according to the SPI emulsions and emulsion gels properties,the increase in the size and content of protein aggregates decreased the oil droplets size and gelling temperature of SPI emulsions.More and/or large protein aggregates helped to form SPI emulsion gels with better viscoelasticity.The microstructures of SPI emulsion gel containing smaller and/or less protein aggregates appeared loose with more holes,with the increase in the protein aggregate size and content,the gel network became denser and more uniform,correspondly,the water-holding capacity of SPI emulsion gels was significantly enhanced.In addition to protein aggregation,the effects of emulsion aggregation on the SPI emulsion gel properties were also investigated.We proposed a novel approach,adding the coagulant in two steps instead of one step for traditional process,to prepare SPI emulsion gel.Before the gelation of SPI emulsion,a small amount of coagulant was added into the emulsion,allowing it to aggregate;And then the remaining coagulant was further added to form a SPI emulsion gel.When compared with the gel prepared by traditional method?one-step without pre-aggregation?,pre-aggregation could decrease the gelling temperature of SPI emulsion and accelerate the gelation process.This acceleration effect was enhanced as the Ca²⁺concentration during pre-aggregation increased.Moreover,pre-aggregation promoted to form stronger SPI emulsion gels with better viscoelastic properties.When the Ca²⁺concentration during pre-aggregation increased from 0 to 7.5 mM,the storage modulus?G'?of the gel increased by 80%,approximately.However,when the Ca²⁺concentration reached to 10 mM,the G'value decreased.Higher Ca²⁺concentration during pre-aggregation?0 to 7.5 mM?resulted in gel structures harder but more brittle,whereas under lower Ca²⁺concentration during pre-aggregation,the gels were softer but tough.The microstructure analysis demonstrated that with the increase in the Ca²⁺concentration during pre-aggregation from 0 to 7.5 mM,the microstructures of SPI emulsion gels became denser,more uniform and compact.For the gel pre-aggregated at 10 mM Ca²⁺,the structure was coarse,porous and discontinuous.These results indicated the importance of the structural changes in SPI emulsion gels on their properties.The water-holding capacity of SPI emulsion gels with stronger,denser and more compact structure was higher,whereas a loose,porous and weak gel structure tended to bond water to a less extent.At last,effects of the cooperation of different coagulants on SPI emulsion gel properties was studied.Instead of single CaSO₄ as the coagulant,the coagulant complex containing low concentration of Mg²⁺?e.g.,5 mM?could form uniform and dense SPI emulsion gels,which had better viscoelasticity and water-holding capacity.However,excessive Mg²⁺made protein aggregates large but coarse,leading to the formation of coarse SPI emulsion gel networks with more holes.As a result,the viscoelasticity and water-holding capacity of the gels decreased.On the other hand,TGase induced acyl-transfer reaction between?-carboxyamide groups of glutamine residues and?-amino group of lysine in proteins decreased the surface hydrophobicity and Emulsifying property of SPI,resulting in the coalescence of oil droplets,therefore,the oil droplets size increased.The pre-treatments of 0 to 3 U TGase did not affect the elasticity of SPI emulsion gels significantly,whereas pre-treatment of 5 U TGase on the SPI emulsion decreased the gel molulus.In addition,enzyme treatment significantly increased the fracture stress,fracture strain and water-holding capacity of SPI emulsion gels.The results of force analysis within the gel network revealed that hydrophobic interactions and S-S bonds are the main forces to maintain the structure of SPI emulsion gel networks,but the amount of hydrophobic bonds involved in SPI emulsion gels pre-treated by TGase were significantly lower than those in gel without TGase pre-treatment,indicating that TGase treatment could limit the formation of hydrophobic bonds.